I added a RGBAToHTMLColor and made some minor changes.

Added a vtkStdString RGBAToHTMLColor(const vtkColor4ub & rgba)
method  which given a vtkColor4ub instance as input color returns
a valid HTML color string in the `rgba(r, g, b, a)` format.

Change-Id: I490d173c2fc2d66869cda4d0db7ae6f632607818